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Mark 55 motors came in several series (Mark 55 / 55E-1) (Mark 55 / 55E-2) (Mark 55 / 55E-3) (Mark 55 / 55E-4) and (Mark 55 A).
These (Mark 55 / 55E-1 Mark 55 / 55E-2) came with three different flywheels. Only one has the same part number (208-998A1) as the MK 50E. These models also came with only one crankshaft (400-3). This is the only crankshaft used on the MK 50.
These (Mark 55 / 55E-3 Mark 55 / 55E-4) came with no less than five different flywheels, two with the small taper and three with the large. None of them have the same part number (208-998A1) as the MK 50. While a small taper flywheel from this series may fit the Mk 50, it very well may have the wrong tooth count. Large taper flywheel and crankshaft (400-1319) started with ser # 1094866 on Mark 55E-3 engines.
The compatibility stops with the Mk 55/55-E 1&2 series. Mark 55 A and 58 don’t even figure into the equation. They all have the large taper flywheel and crankshaft (400-1319).
The bottom line seems to be the Mk 50 had only one crankshaft and one electric start flywheel. This, small taper, crankshaft was also shared with early Mark 55 models. Some early Mk 55 motors also had the sand cast blocks, which would mean no mounting holes for the starter.
The Mk 50 electric start flywheel is also shared with the early Mk 55 and those Mk 30 models with the small taper crankshaft. My Mk 30 has the small flange, manual start flywheel, with the manual start ratchet cast into the top of it. It fits and functions as a manual start on my Mk 50.
I have not been able to find the article I previously read about the starters and flywheels but as I recall there is a one-tooth difference between the large flange and the small flange flywheels and their corresponding starters. So a starter used on an early Mk 55, with small flange and a starter used on a Mk30, with small flange would have the correct number of teeth on the bendix drive but not necessarily have the same mounting flange or choke solenoid mount to be used on a Mk 50.